Engine out cold-start hydrocarbon emissions generated before light-off of an exhaust system catalytic converter may contribute a large percentage of the total exhaust hydrocarbon (HC) emissions. Accordingly, engine exhaust systems may utilize hydrocarbon retaining devices, such as hydrocarbon traps, to retain cold start emissions for later reaction (HC storing), or for recirculation through the engine (HC purging).
One example system for storing and purging hydrocarbon emissions is shown by Tanaka et al. in U.S. Pat. No. 5,410,875. Herein, a HC trap is positioned in a first exhaust bypass passage for storing HC emissions during engine cold start. When exhaust temperatures reaches a desired threshold, the stored HCs are purged into an exhaust main passage via a second bypass passage.
However, the inventors have recognized several potential issues with such a system. As one example, when returning exhaust gas to a location upstream of a catalyst in the main passage, the system flows gases against exhaust pressure. As such, this may reduce the efficiency with which the gases are purged into the main exhaust passage. As another example, a large number of valves and passages are used to ensure the correct flow of purged HCs against the exhaust pressure. These include multiple valves and passages required to direct exhaust flow from the exhaust bypass passage to the exhaust main passage, as well as valves and passages required to ensure manifold vacuum and pull ambient air for trap purging. As such, the need for additional hardware adds cost and complexity to the system
Thus, in one example, some of the above issues may be addressed by a method of operating an engine having a hydrocarbon retaining system and an emission control device coupled to an engine exhaust, the engine exhaust comprising a venture. The method may comprise, during a storing condition, routing exhaust gas through the venturi without generating a venturi action, and then to the hydrocarbon retaining system, while bypassing the emission control device, to store hydrocarbons in the hydrocarbon retaining system. The method may further comprise, during a purging condition, routing exhaust gas through the venturi while generating venturi action, then to the emission control device, and then to the hydrocarbon retaining system, to purge stored hydrocarbons, wherein a flow of purged hydrocarbons is drawn back to the venturi via venturi action.
In one example, a venturi with at least three openings may be located in an engine exhaust conduit. Specifically, the venturi may be located at a junction between a bypass conduit wherein a HC trap may be positioned and a main exhaust conduit. At least one of the openings may be located in the narrow region of the venturi and may provide a passage into the bypass conduit. Similarly, at least one of the openings may be located in a broad region of the venturi and may provide a passage into the main exhaust conduit, towards an emission control device and/or the atmosphere. During a storing operation, flow through the venturi may be adjusted to disable a venturi action, for example by restricting flow through an opening in the broad region of the venturi. Consequently, exhaust gas may enter the bypass conduit, through the opening in the narrow region of the venturi, and exhaust HCs may be trapped. In contrast, during a purging operation, flow through the venturi may be adjusted to enable a venturi action, for example by enabling flow through the previously restricted opening in the broad region of the venturi. The resulting low pressure region created in the venturi (that is, the venturi action generated) may enable exhaust to be drawn in from the bypass conduit and HC trap towards the venturi. Specifically, the exhaust may be drawn in through the opening in the narrow (e.g., lower pressure) region of the venturi. The exhaust may then flow through an opening in the broad region of the venturi into the main exhaust conduit and a downstream emission control device before being vented to the atmosphere.
In this way, the direction of flow through a venturi may be alternated by selectively (e.g., sequentially in one example) restricting and un-restricting flow to/from at least one opening of the venturi. By regulating flow through the venturi opening, a venturi action may be sequentially increased and decreased (e.g., enabled and disabled), thereby selectively varying the direction of an exhaust flow. By enabling a venturi action, purged HCs may be drawn into a main exhaust conduit without flowing against exhaust pressure. That is, HC purging may be enabled with a reduced amount of additional hardware (such as valves and passages) to provide proper flow (although additional valves may be used, if desired). In this way, the use of a venturi may be extended to engine exhaust after-treatment even when the venturi action is disabled.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.